Two-wire current transmitter employing a stabilized current source in one electrical circuit containing control means in series therewith for controlling a direct current amplifier contained in a parallel circuit

ABSTRACT

A transmitter controls the flow of load current through only two wires in accordance with the magnitude of a variable condition. Within the transmitter the current follows two paths, one portion of the current flowing through a direct current amplifier and the remaining portion flowing through a stabilized current source which provides the sole energization for certain components, including a condition responsive bridge network, in order to operate the DC amplifier in response to the condition sensed by the bridge network. The current recombines to constitute the load current that is indicative of the magnitude of the variable condition.

United States Patent [151 3,675,122

Rose 1 July 4, 1972 54] TWO-WIRE CURRENT TRANSMITTER [56] References cmEMPLOYING A STABILIZED CURRENT I SOURCE IN oNE ELECTRICAL m TENTSCIRCUlT CONTAINING CONTROL I er, r. e MEANS IN SERIES THEREWITH FOR3,517,556 6/ I970 Barker "73/3 62 CONTROLLING A DIRECT CURRENT AMPLIFIERCONTAINED IN A PARALLEL CIRCUIT Robert C. Rose, Minneapolis, Minn.

Filed: Oct. 27, 1969 Appl. No.: 869,687

Inventor:

Assignee:

Primary Examiner-Rudolph V. Rolinec Assistant Examiner-David M. Carter 7I Attorney-Bugger, Peterson, Johnson & Westman [57] ABSTRACT recombinesto constitute the load current that is indicative of the magnitude ofthe variable condition.

13 Claims, 2 Drawing Figures CURRENT SOURCE PATENTEDJuL 4 m2 3.675.122

SHEET 1 OF 2 52 J W J 36 CURRENT SOURCE F r I iv 0c cnocz wgn CHOPPERDEMODULATOR AMP 40 I 4\ 42 52 30 BRIDGE so I52 FIG! TWO-WIRE CURRENTTRANSMITTER EMPLOYING A STABILIZED CURRENT SOURCE IN ONE ELECTRICALCIRCUIT CONTAINING CONTROL MEANS IN SERIES TIIEREWITH FOR CONTROLLING ADIRECT CURRENT AMPLIFIER CONTAINED IN A PARALLEL CIRCUIT BACKGROUNDOFTHE INVENTION 1. Field of the Invention This invention relates generallyto a measuring system embodying .a current transmitter having only twowires connected in circuit therewith which carry the direct currentpower to the transmitter and also which current is adjusted to representthe magnitude of the change in a variable condition, and pertains moreparticularly to a current transmitter utilizing a stabilized currentsource which provides accurate control of the load current as a functionof the variable condition being measured while encountering widevariation in ambient temperature environments and applied voltagelevels.

2. Description of the Prior Art The state of the art has been generallydealt with in the patent application of Earl A. Grindheim, Ser. No.661,988, filed Aug. 21, I967 now abandoned and continuation Ser. No.14,748 has been substituted therefore, for Two-Wire Current TransmitterResponsive to a Resistance Sensor Input Signal and assigned to the sameassignee as the present invention. While the transmitting systemdescribed in the Grindheim application has performed admirably well, theinvention herein illustrated is intended to be an improvement over theGrindheim transmitter in certain respects that will hereinafter becomemanifest.

SUMMARY OF THE INVENTION One object of the invention is to provide atwo-wire transmitter that will require only a small amount of power forits operation, this being accomplished by reducing both the voltage andcurrent needed by the transmitter.

Another object of the invention is to provide a two-wire transmitterthat will accommodate a wide range of supply voltages. In this regard,although the transmitter will function with a low supply voltage, theseries load resistance employed by the user may be as great as a fewthousand ohms and should it become shorted the transmitter mustwithstand the full supply voltage. While a system can be designed tooperate with high supply voltages, the system then encounters difficultywhen the voltage is low. It is an aim of the present invention toprovide a transmitter that will operate on approximately 17 volts andyet which will withstand voltages approximately 3- 15 times the ratedvalue.

A very important object of the invention is to provide a highlystabilized current source, both as to voltage and temperature. Morespecifically, the current source herein illustrated is stable over a 25to l voltage ratio, is capable of withstanding a ratio of 35 to l, andis temperature compensated from 40 to +85 C.

Still further, an object of the invention is to provide a current sourcethat does not require the selection and utilization of special resistorsor other expensive components, thereby enabling the entire transmitterto be manufactured at a relatively low cost.

Yet another object of the invention is to provide atransmitting systemin which virtually the entire current source output is made available tothe condition responsive bridge.

Generally, an overall object of the invention is to provide a currenttransmitter that will provide an accurate indication of the magnitude ofthe variable condition being measured and which will do so where thetransmitter and load are at widely separated locations and which willremain accurate despite large load resistance variations.

current portions, however, are subsequently recombined and the totalcurrent is closely regulated by the transmitter to permit meters,recording equipment and the like to be accurately operated thereby.

Consequently, a general object of the invention is to provide atransmitter that controls a current that is representative of acondition being measured, doing so in a highly accurate mannerirrespective-of voltage, load resistance and ambient temperature changesand also with a minimal amount of power, and having the currentprecisely proportional to the quantity being measured even though thatquantity may vary over a relatively wide range. Hence, the transmitterherein il- A still further object of the invention is to provide atransmitting system where the current flowing through the sensorlustrated possesses a number of long sought after attributes whichcollectively contribute to a reliable and accurate measuring system.

Quite briefly, the invention envisages a transmitter that is connectedin series-with only two wires through which the load current flows, theload current being furnished by an external power source. Thetransmitter is operated by the load current and functions to vary themagnitude thereof in accordance with the particular condition beingsensed. Two current paths are provided within the transmitter, one pathbeing through a DC amplifier, the remaining current portion flowingthrough the second path. The current portion traversing the second pathis used for energizing the condition responsive bridge network and alsothe alternating current amplifier that is responsible for controllingthe DC amplifier, which is in the first path, in accordance with theamount of variation in the condition being measured. Note also that thecurrent through the DC amplifier path also flows through a portion ofthe bridge network as described in said Grindheim application. Thecurrent through the bridgenetwork, and also the AC circuitry, isstabilized or regulated to a high degree by zener diodes that derivetheir current from a current source that can withstand relatively highvoltages. The constant current source is also temperature;compensated soas to enhance its accuracy and in turn increase the overall precision ofthe transmitting system.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagram, for the most partin block form, illustrating an exemplary transmitter constructed inaccordance with the teachings of the invention, and FIG. 2 is a diagram,mostly in schematic form, corresponding generally to what appears inFIG. 1 but with the load and DC. power supply added thereto.

DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, itwill be seen from FIG. 2 that a load has been indicated by the referencenumeral 10. The load 10 may be a milliammeter, a recorder, or anyutilization device. The load 10 is connected in series with a directcurrent power supply 12 which provides all of the power utilized by thecircuitry hereinafter described. It will be understood that a singlepair of wires 14 and 16 constitute the only conductors that both supplypower to the transmitter hereinafter discussed and also carry the signalcurrent. Stated somewhat differently, the two wires 14, 16 constitutethe signal channel and the load current flowing through these wires isthus indicative of the amount of variation that takes place as far asthe particular condition to be measured is concerned. It is also to berecognized that the two wires l4, 16 are of indeterminate length.Actually, the load 10 and the power supply 12 may be located at onevantage point and the transmitter hereinafter described at a remotedistance that can be quite appreciable.

The transmitter, which will be designated generally by the letter T, hasa pair of terminals 18 and 20 that are connected to the ends of the twowires 14 and 16. Included in the transmitter T is a diode 22, the diode22 functioning to safeguard the transmitter circuitry should the userthereof inadvertently reverse the polarity of the power supply 12 wheninitially making up the connections. It is to be noted that the powersupply 12 shown in FIG. 2 should be connected so as to apply a positivepolarity to the terminal 18 and a negative polarity to the terminal 20.Whereas the anode of the diode 22 is connected directly to the terminal18,.the cathode thereof is connected to a junction labeled 24. Althoughall of the current flowing through the load passes through the wires 14and 16, the load current separates at the junction 24 so that ittraverses two parallel paths or circuits.

The first path or circuit that one portion of the load current followsis via a conductor 26 and a current limiting resistor 28 to a directcurrent amplifier 30. Although the direct current amplifier 30-is notunique, it will be well to reserve a more detailed description thereofuntil after various circuit sections constituting the transmitter T (ofwhich the amplifier 30 is a part) have been referred to. In this way, ageneral picture can be presented in FIG. 1 with respect to the variousfunctions that occur within the transmitter T and the particularcomponents contained in the various circuit sections that provide thefunctions will be considered with greater particularity hereinafter inconjunction with FIG. 2. It will be helpful, though, to note that aconductor 32 is connected to the output terminal labeled 33 of theamplifier 30 and carries the current flowing through the particular pathor circuit that includes the amplifier 30 so that it its recombined withthe current flowing through the second parallel path or circuit thatwill nowbe described.

The second path or circuit over which the remaining portion of the loadcurrent flows, that is the portion not traversing the first circuitwhich contains the amplifier 30, includes a conductor 34 leading fromthe junction 24 to a constant current source 36 which plays a veryimportant role in the practicing of the invention. As will be mademanifest as the description progresses, the source 36 stabilizes'thecurrent passing therethrough irrespective of voltage and temperaturevariations to which the transmitter T may besubjected.

A conductor 38 extends from the output terminal 39 of the constantcurrent source 36 to a chopper drive or multivibrator 40. The sameconductor 38 also supplies current from the source 36 to an alternatingcurrent carrier amplifier 42. An additional conductor 44 connects theother sides-of the circuit sections 40 and 42, thatis, the other sidesof the chopper drive and the AC amplifier, together. In other words, thecircuit sections 40 and.42 are regulated by a zener diode 46, the diode46 being connected between the conductors 38 and 44 and carrying aportion of the stabilized current provided by the current source 36.

A chopper or modulator 48 is connected to a condition responsiveresistance bridge network 50. The chopper 48 controls the carrieramplifier 42 so that the amplified output therefrom is delivered to adetector or demodulator 52 which in turn controls the first stage of thepreviously mentioned DC amplifier 30, doing so via a conductor 54 and acoupling resistor 56. Although the network 50 has not as yet beendescribed in detail, it will be discerned that another conductor 58extends from the bridge network to the terminal 20, this particularconductor 58 carrying the recombined current from the direct currentamplifier 30 which is delivered via the conductor 32 and the stabilizedcurrent that has passed through the bridge network 50 as provided by theconstant current source 36.

Considering now the various circuit sections in greater detail,attention is first directed to the DC amplifier 30. C01- lectively, theamplifier includes a PNP transistor 62 and an N PN transistor 64 whichtogether provide relatively high current gain. A zener diode 60 isconnected so as to provide a current limiting function. Still anotherNPN transistor 66 is included in the amplifier 30, this constituting thefirst stage of the amplifier and having its base connected to the outputside of the demodulator 52 through the resistor 56 and conductor 54. Thecollector of the transistor 66 is connected to the current source 36through a resistor 68 and its emitter is connected to the conductor 32at the junction 33. The signal derived from the demodulator 52 is thusused to control the amplifier 30 in accordance with the magnitude of itsdetected output signal.

Of appreciable importance to the invention is the makeup of the constantcurrent source 36. In this regard, it will be discerned that the currentsource 36 comprises a first PNP transistor 70, a second NPN transistor72 and a third NPN transistor 74, the collector-emitter of thetransistor 74 having capacitor 75 connected thereacross to providedynamic stability. A resistor 76 is connected across the emitter andbase of the transistor 70. However, in order to cause conduction of thetransistor 70 when only a relatively low voltage is applied via thejunction 24, there is a resistor 78 connected between the emitter andcollector of the transistor 70 and through resistor 80 to the base ofthe transistor 72, an initial current path can be established which iseffective in turning on the transistor 72 which otherwise would remainnon-conductive. The transistor 72 has a resistor 82 connected betweenits collector and the base of the transistor 70. In this way, thecurrent flowing due to the initial conduction of the transistor 72 isinstrumental in causing transistor 70 to commence conduction.

The principal current path during operation of the constant currentsource 36 is through the transistor 70 and through a pair of diodes 84and 86. It should be explained at this time that the diodes 84, 86 donot start conducting immediately but require a voltage buildup such thatthe difference across the two will approximate one volt, that is,approximately 0.5 volt across each in order to render them conductive.This insures that transistor 72 will commence conduction.

Further, included in the current source 36 is a temperature compensatingbridge network composed of a resistor 88 shunted by a thermistor 90. Itwill be perceived that one end of the resistor 88 is connected to thecathode of the diode 86 and also to the emitter of the transistor 72 ata junction denoted by the reference numeral 87. The other end of theresistor 88 is joined to a resistor 92 and the opposite end of theresistor 92 is attached to a resistor 94, this last resistor 94 beingshunted by a thermistor 96. Briefly, the thermistor 90 is efiective asfar as its corrective effort is concerned from approximately -15 to C.Below 1 5 C, without the thermistor included, the current would increasequite substantially and it is through the coaction provided by the twothermistors 90 and 96 that excellent temperature compensation is derivedover the range of from approximately 40 to +8 5 C.

The base of the transistor 74 is connected to the emitter of thetransistor 72 and also to the cathode of the diode 86. As explainedabove, and which is evident from the schematic diagram, this arrangementresults in a connection of the base of the transistor 74 to the junction87. Thus, should the current delivered to the resistance network, whichis composed of the various resistors 88, 92, 94 and the thermistors 90,96, increase, the voltage will increase at the junction 87. The functionof the'transistor 74 is to react to the changing voltageat the top ofthe resistance network, that is at the junction 87 and to limit it. Inthis way, if the current through the resistance network 88-96 shouldincrease, then the voltage will rise slightly at the junction 87 tocause the transistor 74 to increase conduction by reason of the slightlyincreased positive voltage that is thus applied to its base. However,owing to this happening, the base of the transistor 72 becomes morenegative with the consequence that the conduction of this particulartransistor is decreased. Therefore, less current is supplied to thejunction 87 through the transistors 70 and 72 and the result is that thecurrent at the output junction 39 of the current source 36, ismaintained relatively constant. Temperature variations that affect thebase-emitter volt-drop of transistor 74 are tracked by the resistancenetwork 88-96 such that at constant current the network's volt-dropmatches the temperature coefficient of transistor 74. Thus, changingtemperatures do not change the output current. When the temperature towhich the current source is subjected increases, then the re sistance ofthe above-alluded to network decreases.

As far as the chopper drive or multivibrator 40 is concemed, it isconventional. In this regard, it will be observed that it includes apair of NPN transistors 100 and 102. The bases of these transistors 100,102 are interconnected through cross-coupling capacitors 104 and 106that cause the transistors 100 and 102 to alternate back and forth asfar as their conductive states are concerned. The chopper drivemultivibrator 40 is energized, as-can be seen from the schematicdiagram, by the difference in potential existing between the conductors38 and 44.

Also energized from the potential difference between the conductors 38and 44 is the AC carrier amplifier 42. In the illustrative instance, itwill be discerned that the amplifier 42 consists of two stages, beingcomposed of a first NPN transistor 108 and a second NPN transistor 110.The first transistor 108 is coupled to the chopper or modulator 48 via acapacitor ll2, whereas the collector of this transistor 108 is coupledto the base of the transistor 110 os the second amplifying stage bymeans of a capacitor 114.

Passing now to a detailed description of the chopper of modulator 48, itwill be seen that this circuit section is comprised of two field efiecttransistors 116 and 118, the gates of the two field effect transistorsbeing coupled in one instance by a capacitor 120 connected to thecollector of the transistor 100 constituting part of the chopper driveor multivibrator 40. in similar fashion, the gate of the other fieldeffect transistor 118 is coupled to the collector of the transistor 102of the chopper or multivibrator 40 through the agency of a capacitor122. Thus, the chopper 48 performs its chopping function by having thetransistors 116 and 118 alternately gated into a conductive statethrough the agency of the chopper drive or multivibrator 40. Statedsomewhat differently, when the transistor 100 of the chopper ormultivibrator 40 is non-conductive, the collector thereof will be atvirtually the same potential as the conductor 38, whereas the thenconducting transistor 102 will have its collector at virtually thepotential of the conductor 44. Hence, with the staged voltage applied tothe gate of the field effect transistor 116, this particular fieldeffect transistor of the chopper or modulator 48 will be nonconductiveand with the lower collector potential applied to the gate of the otherfield effect transistor 118 of the chopper or modulator 48 it will beconductive. As is typical with modulators, the switching action thatoccurs will in turn drive the respective field effect transistors 116and 118 into alternately conductive and non-conductive states.

The condition responsive resistance bridge network 50 is utilized forsensing any changes in the particular variable condition to be measured.In achieving this aim, the network 50 comprises a fixed resistor 124connected to one end of a fixed resistor 126, the junction of theresistors 124, 126 being connected to the previously mentioned conductor44 through an adjustable resistor 128. The resistor 126 has its otherend connected to one end of a potentiometer 130 that has its other endin turn connected to a terminal 132. There is a corresponding terminal134 so that dummy leads 136 connected between the terminals 132, I34constitute a compensation loop intended to offset any resistanceappearing in the leads connected to terminals 148 and 150 other thanthat of resistor 152. The potentiometer 130 has a wiper arm 138 that isconnected to the source of the field effect transistor 118 contained inthe chopper or modulator 48. Another fixed resistor 140 has one endattached directly to the terminal 134 of the compensating loop and itsother end connected to the junction of two fixed resistors 142 and 144.The resistor 142 is connected to the conductor 32 that extends from theoutput side of the DC amplifier 30. The resistor 144, as will be soonmade clear, functions as a feedback resistor. However, there is astabilizing diode 146 connected between the other end of the resistor142, that is the end remote from the end thereof that is connected tothe junction of the resistors 140 and 144, and the diode 146 is also, asis obvious, connected to said conductor 32. The diode 146 is of thezener type and together with the resistor 142 maintains a referencevoltage on the wiper arm 138 of the potentiometer 130. The resistor 144,functioning as a feedback resistor as explained above, has the other endthereof, that is the end remote from the resistors 140 and 142,connected to the terminal 20. The temtinal 20, as is evident from thediagram, is likewise connected to a terminal 148, the temtinal 148having a companion terminal 150. It is between the two terminals 148,150 that the sensing element labeled 152 is connected. Thus, theparticular sensing element, whether it be a temperature sensitiveresistor or some other element capable of providing a variable voltagein accordance with the condition being sensed can be readily changed,the terminals 148, 150 affording the opportunity for substitution. Theterminal 150 not only connects directly to one end of the previouslymentioned resistor 124 but also is connected to the drain of the fieldeffect transistor 1 l6.

Describing with greater particularity the demodulator or detector 52, itwill be seen that this demodulator includes a pair of field efiecttransistors 156 and 158. As with the chopper or modulator 48, the gatesof the two field effect transistors 156, 158 are tied or coupled to thecollectors of the transistors and 102 of the chopper drive ormultivibrator 40 by way of capacitors 160 and 162, respectively. The twosources of the transistors 156 and 158, these transistors being of thefield effect variety as mentioned above, are connected together and arein turn connected to the collector of the transistor belonging to thesecond stage of the AC carrier amplifier 42, the connection or couplingbeing through a capacitor 164 and a resistor 166. In this way, theoutput from the AC carrier amplifier 42 is demodulated by the fieldeffect transistors 156 and 158 in correspondence with the switchingaction taking place at the chopper drive or multivibrator 40. When thefield effect sistor 56. On the other hand, when the other transistor 158is rendered conductive, there is a path provided through a diode 164,the diode 164 being forwardly biased by means of a resistor 166. Inother words, the resistor 166 and the diode 164 are serially connectedbetween the conductors 38 and 32.

Having presented the foregoing description, the general manner in whichmy transmitter T operates should be understood. The input current, thisbeing the load current through the load 10 and the serially connectedwires 14 and 16 passes through the current terminal 18 and then throughthe diode 22. It is at the junction 24 that this load current separatesafter having passed through the protecting diode 22. A variable portionof the current flows through the conductor 26, the resistor 28 and iscontrolled by the DC amplifier 30. A fixed amount is allowed to passthrough the current source 36. It should be kept in mind that thecurrent through these two circuits recombines. However, at this time itwill be emphasized that the current from the current source 36 isregulated as far as voltage and temperature is concerned and serves toenergize the multivibrator 40, the carrier amplifier 42 and one stage ofthe DC amplifier 30, this being the stage that includes the transistor66. Also, the current that flows through the current source 36 energizesthe bridge network 50. However, the current recombines and the totalcurrent flowing through the terminal 20 is the same as that entering thetransmitter T via the tenninal 18.

While the network 50 functions in the same manner as the resistancenetwork shown in said Grindheim application, it should be pointed outthat any unbalance of the bridge voltage between terminals and 138caused by either a change in the variable condition sensed by the sensoror sensing element 152 or a difference between the desired current andthe actual amount will be reflected in the output from the chopper 48.Since the output from the chopper 48 is amplified by the AC carrieramplifier 42, the amplified signal, which is representative of themagnitude of the error sensed by the bridge 50, will cause the value ofthe signal from the demodulator 52 to control the input to the DCamplifier 30. Consequently, the load current, this being the sum of thecurrent passing through the first and second parallel paths (one paththrough the amplifier 30 and the other path through the constant currentsource 36) will represent the actual magnitude of the variable conditionthat is being sensed. Hence, the current through the load 10, the load10 being in series with the conductors l4 and 16, will be indicative ofthe actual magnitude or value determined by the sensor 152.

Should the current fluctuate in the second path, the second pathincluding the current source 36, then the change in current will bereflected in the signal from the demodulator52 and the DC amplifier at30 will be amplifying an incorrect signal. Hence, even without preciseregulation or stabilization provided by the current source 36, the loadcurrent will be truly representative of the magnitude of the variablecondition.

Quite briefly, the constant current source 36 operates so 1 that afterits initial start up, there is a primary current path from the emitterof the transistor 70 to the collector thereof, through the two diodes 84and 86, and then through the resistor network comprising the variousresistors and thermistors 88-94. The purpose, as already explained, ofthe resistor 78 is solely for turning on the circuitry, for if theresistor 78 is not included, it is possible that the lack of leakagethrough the transistor 70, especially when cold, would not effeet theturning on of the other transistors 72 and 74. Hence, the currentleaking through the resistor 78 is blocked by the diodes 84 and 86 andmust follow the path through the resistor 80. Upon entering the base ofthe transistor 72, which exhibits gain and has a collector voltageapplied thereto by virtue of resistors 82 and 76, the collector currentflows through the transistor 72 and simultaneously from the emitter tothe base of the transistor 70. The transistor 70 responds by allowingcollector current to flow down toward the diodes 84 and 86, the circuitthen springing into full operation. The transistor 74 detects the levelof the voltage at the junction to the left of the resistor 88. Shouldthe current increase. the voltage at this junction slightly, there willbe more base current supplied to the transistor 74.'On a currentsplitting basis, as one looks at the base of the transistor 72 and theresistor 80, the resistor 80 having essentially constant current flowingtherethrough, if more current is sent through the collector oftransistor 74, less is available to the base of transistor 72 which willreduce the net current drawn by the entire constant current source 36.

The temperature compensation is provided by the resistance network88-96, the resistance thereof being reduced as the temperature thereofis increased. The net result, though, is that irrespective of thevoltage at the junction 24 or the temperature to which the currentsource 36 is subjected, the output therefrom will be substantiallyconstant and this allows the proper sensing to take place withoutintroducing errors. Thus, the amplification provided by the DC amplifier30 is controlled so that the total load current, this being therecombined current from the circuit including the amplifier 30 in thecircuit including the source 36, will be indicative of the value of thevariable condition being sensed at 152.

I claim:

1. A current transmitter comprising a pair of current terminals, firstand second circuits connected in parallel with each other and in serieswith said terminals so that the current flowing through said first andsecond circuits also flows through said terminals, said first circuitincluding a direct current amplifier and said second circuit including asingle constant current source, circuit means connected in series withsaid source so that all of the current flowing through said singleconstant current source flows through said circuit means for producingan alternating current derived entirely from said current flowingthrough said constant current source, means connected to saidalternating current means for controlling said alternating current meansin accordance with the. magnitude of a variable condition, and meansconnected to said alternating current means for controlling said directcurrent amplifier, whereby the current flowing through said terminals isrepresentative of the value of said variable condition.

2. A current transmitter as set forth in claim 1 in which saidalternating current means includes an oscillator and alternating currentamplifier in parallel therewith, said means for controlling saidalternating current means in accordance with a variable conditionincluding a chopper connected between said oscillator and alternatingcurrent amplifier so that said oscillator drives said chopper and saidalternating current amplifier amplifies the output from said chopper,said means for controlling said alternating current means controllingthe current to said chopper so that the output therefrom to saidalternating current amplifier is changed in response to any change inthe magnitude of said variable condition;

3. A current transmitter as set forth in claim 2 in which said meansresponsive to said alternating current means includes a demodulatorconnected between said alternating current amplifier and said directcurrent amplifier for controlling said direct current amplifier inaccordance with the output from said demodulator. v

4. A current transmitter as set forth in claim 1 in which saidalternating current controlling means includes a resistance bridgehaving a temperature sensitive element therein.

5. A current transmitter comprising a pair of current terminals, firstand second circuits connected in parallel with vide a first path, diodemeans and a temperature compensating resistance network, said diodemeans being connected to said first path between said first and thirdtransistors and to said resistance network, the emitter. and collectorof said second transistor forming a second path in parallel with theemitter-collector path of said first transistor, the respective bases ofsaid first and third transistors being connected to spaced points onsaid second path and the base of said second transistor being connectedto a point between said first and third transistors in said first path,said first transistor providing a primary current path therethrough whenconducting which continues through said diode means and compensatingnetwork, said current transmitter further including means connected inseries with said source for producing an alternating current, meansconnected to said alternating current means for controlling saidalternating current means in accordance with the magnitude of a variablecondition, and means connected to said alternating current means forcontrolling said direct current amplifier, whereby the current flowingthrough said terminals is representative of the value of said variablecondition.

6. A current transmitter comprising a pair of current terminals, firstand second circuits connected in parallel with each other and in serieswith said terminals so that the current flowing through said first andsecond circuits also flows through said terminals, said first circuitincluding a direct current amplifier and said second circuit including aconstant current source comprising first, second and third transistors,said second and third transistors being of NPN conductivity with saidfirst transistor being of PNP conductivity and each transistor having anemitter, collector and base, diode means, the emitter of said firsttransistor being connected to one of said current terminals and thecollector thereof being connected to one side of said diode means, atemperature compensating resistance network connected to the other sideof said diode means to provide a primary current path through theemitter-collector of said first transistor when conductive, said diodemeans and said resistance network, means connected to the collector ofsaid first transistor and to the base of said second transistor forinitially rendering said second transistor conductive so as to causesaid first transistor to conduct to provide said primary current paththrough said diode means and resistance network, the collector of saidsecond transistor being connected to the said one current terminal andthe emitter of said second transistor being connected to the junction ofsaid diode means and said network, the base of said first transistoralso being connected to said one current terminal, said secondtransistor providing a secondary current path through its collector andemitter, said diode means and resistance network, the base of said thirdtransistor being connected to the junction of said diode means and saidnetwork to render said third transistor responsive to the voltage at thejunction of said diode means and network for reducing the amount ofconduction of said third transistor through its collector-emitter pathto reduce the total current through said source, said currenttransmitter further including means connected in series with said sourcefor producing an alternating current, means connected to saidalternating current means for controlling said alternating current meansin accordance with the magnitude of a variable condition, and meansconnected to said alternating current means for controlling said directcurrent amplifier, whereby the current flowing through said terminals isrepresentative of the value of said variable condition.

7. A current transmitter as set forth in claim 6 in which saidresistance network includes first, second and third resistors connectedin series, a first thermistor in parallel with said first resistor and asecond thermistor in parallel with said third resistor of saidresistance network.

8. In a two-wire current transmitter, a constant current source havinginput and output junctions, the source comprising first, second andthird transistors, each having a base, collector and emitter, diodemeans, a temperature compensating resistance network, the emitter ofsaid first transistor being connected to said input junction and thecollector thereof being connected to the anode side of said diode means,the cathode side of said diode means being connected to one side of saidresistance network and the other side of said resistance network beingconnected to said output junction, a first resistor connected betweenthe emitter and collector of said first transistor, a second resistorconnected between the collector of said first transistor and the base ofsaid second transistor and also to the collector of said thirdtransistor, the emitter of said second transistor being connected to thejunction of said diode means and network and also to the base of saidthird transistor, the emitter of said third transistor being connectedto said output junction, a third resistor connected between the emitterof said first transistor and the base thereof, a fourth resistorconnected between the base of said first transistor and the collector ofsaid second transistor, whereby a primary current path exists throughthe emitter-collector of said first transistor, said diode means andsaid resistance network when said first transistor is conducting.

9. The combination of claim 8 in which said resistance network includesfirst, second and third serially connected resistors, a first thermistorconnected in parallel with said first resistor and a second thermistorconnected in parallel with said third resistor of said resistancenetwork.

10. A current transmitter comprising a pair of current terminals, adirect current amplifier connected between said terminals through whichamplifier a variable portion of the current flowing through saidterminals passes and is controlled, a constant current source connectedbetween said terminals and in parallel with said amplifier through whichsource a second portion of the current flows so as to be regulatedthereby, a chopper drive and alternating current carrier amplifierconnected in parallel with each other and in series with said currentsource, a chopper connected between said chopper drive and alternatingcurrent amplifier, a condition responsive bridge network for controllingsaid chopper in accordance with the magnitude of a variable condition,said bridge network being in series with said current source and inseries with said alternating current amplifier to cause the currentsupplied by said current source to recombine with the current throughsaid direct current arnglifier, and a demodulator havin its inputconnected to sm alternating current carner amp ifier and its outputconnected to said direct current amplifier for controlling said directcurrent amplifier, whereby the current through said terminals will beindicative of the amount of variation occurring in the variablecondition.

11. In a current transmitter, a constant current source hav ing inputand output junctions, the source comprising first, second and thirdtransistors, each having a base, collector and emitter, diode means, atemperature responsive resistance network, the emitter of said firsttransistor being connected to said input junction and the collectorthereof being connected to the anode side of said diode means, thecathode side of said diode means being connected to one side of saidresistance network and the other side of said resistance network beingconnected to said output junction, a first resistor connected betweenthe emitter and collector of said first transistor, a second resistorconnected between the collector of said first transistor and the base ofsaid second transistor and also to the collector of said thirdtransistor, the emitter of said second transistor being connected to thejunction of said diode means and network and also to the base of saidthird transistor, the emitter of said third transistor being connectedto said output 40 junction, whereby a primary current path existsthrough the emitter-collector of the first transistor, the diode meansand said resistance network when said first transistor is conduct- 12.The combination of claim 11 in which said resistance network includesfirst, second and third serially connected resistors, a first thermistorconnected in parallel with said first resistor and a second thermistorconnected in parallel with said third resistor of said resistancenetwork.

13. The combination set forth in claim 11 including means in series withsaid constant current source and energized solely thereby for providinga control signal in accordance with the magnitude of a variablecondition, and a direct current amplifier in parallel with said constantcurrent source and said means in series with said constant currentsource and connected to said constant current source so that saidcontrol signal controls said direct current amplifier.

1. A current transmitter comprising a pair of current terminals, firstand second circuits connected in parallel with each other and in serieswith said terminals so that the current flowing through said first andsecond circuits also flows through said terminals, said first circuitincluding a direct current amplifier and said second circuit including asingle constant current source, circuit means connected in series withsaid source so that all of the current flowing through said singleconstant current source flows through said circuit means for producingan alternating current derived entirely from said current flowingthrough said constant current source, means connected to saidalternating current means for controlling said alternating current meansin accordance with the magnitude of a variable condition, and meansconnected to said alternating current means for controlling said directcurrent amplifier, whereby the current flowing through said terminals isrepresentative of the value of said variable condition.
 2. A currenttransmitter as set forth in claim 1 in which said alternating currentmeans includes an oscillator and alternating current amplifier inparallel therewith, said means for controlling said alternating currentmeans in accordance with a variable condition including a chopperconnected between said oscillator and alternating current amplifier sothat said oscillator drives said chopper and said alternating currentamplifier amplifies the output from said chopper, said means forcontrolling said alternating current means controlling the current tosaid chopper so that the output therefrom to said alternating currentamplifier is changed in response to any change in the magnitude of saidvariable condition.
 3. A current transmittEr as set forth in claim 2 inwhich said means responsive to said alternating current means includes ademodulator connected between said alternating current amplifier andsaid direct current amplifier for controlling said direct currentamplifier in accordance with the output from said demodulator.
 4. Acurrent transmitter as set forth in claim 1 in which said alternatingcurrent controlling means includes a resistance bridge having atemperature sensitive element therein.
 5. A current transmittercomprising a pair of current terminals, first and second circuitsconnected in parallel with each other and in series with said terminalsso that the current flowing through said first and second circuits alsoflows through said terminals, said first circuit including a directcurrent amplifier and said second circuit including a constant currentsource comprising first, second and third transistors, each transistorhaving a base, collector and emitter and said second and thirdtransistors being of opposite conductivity type with respect to saidfirst transistor, the emitters and collectors of said first and thirdtransistors being connected in series to provide a first path, diodemeans and a temperature compensating resistance network, said diodemeans being connected to said first path between said first and thirdtransistors and to said resistance network, the emitter and collector ofsaid second transistor forming a second path in parallel with theemitter-collector path of said first transistor, the respective bases ofsaid first and third transistors being connected to spaced points onsaid second path and the base of said second transistor being connectedto a point between said first and third transistors in said first path,said first transistor providing a primary current path therethrough whenconducting which continues through said diode means and compensatingnetwork, said current transmitter further including means connected inseries with said source for producing an alternating current, meansconnected to said alternating current means for controlling saidalternating current means in accordance with the magnitude of a variablecondition, and means connected to said alternating current means forcontrolling said direct current amplifier, whereby the current flowingthrough said terminals is representative of the value of said variablecondition.
 6. A current transmitter comprising a pair of currentterminals, first and second circuits connected in parallel with eachother and in series with said terminals so that the current flowingthrough said first and second circuits also flows through saidterminals, said first circuit including a direct current amplifier andsaid second circuit including a constant current source comprisingfirst, second and third transistors, said second and third transistorsbeing of NPN conductivity with said first transistor being of PNPconductivity and each transistor having an emitter, collector and base,diode means, the emitter of said first transistor being connected to oneof said current terminals and the collector thereof being connected toone side of said diode means, a temperature compensating resistancenetwork connected to the other side of said diode means to provide aprimary current path through the emitter-collector of said firsttransistor when conductive, said diode means and said resistancenetwork, means connected to the collector of said first transistor andto the base of said second transistor for initially rendering saidsecond transistor conductive so as to cause said first transistor toconduct to provide said primary current path through said diode meansand resistance network, the collector of said second transistor beingconnected to the said one current terminal and the emitter of saidsecond transistor being connected to the junction of said diode meansand said network, the base of said first transistor also being connectedto said one current terminal, said second transistor providinG asecondary current path through its collector and emitter, said diodemeans and resistance network, the base of said third transistor beingconnected to the junction of said diode means and said network to rendersaid third transistor responsive to the voltage at the junction of saiddiode means and network for reducing the amount of conduction of saidthird transistor through its collector-emitter path to reduce the totalcurrent through said source, said current transmitter further includingmeans connected in series with said source for producing an alternatingcurrent, means connected to said alternating current means forcontrolling said alternating current means in accordance with themagnitude of a variable condition, and means connected to saidalternating current means for controlling said direct current amplifier,whereby the current flowing through said terminals is representative ofthe value of said variable condition.
 7. A current transmitter as setforth in claim 6 in which said resistance network includes first, secondand third resistors connected in series, a first thermistor in parallelwith said first resistor and a second thermistor in parallel with saidthird resistor of said resistance network.
 8. In a two-wire currenttransmitter, a constant current source having input and outputjunctions, the source comprising first, second and third transistors,each having a base, collector and emitter, diode means, a temperaturecompensating resistance network, the emitter of said first transistorbeing connected to said input junction and the collector thereof beingconnected to the anode side of said diode means, the cathode side ofsaid diode means being connected to one side of said resistance networkand the other side of said resistance network being connected to saidoutput junction, a first resistor connected between the emitter andcollector of said first transistor, a second resistor connected betweenthe collector of said first transistor and the base of said secondtransistor and also to the collector of said third transistor, theemitter of said second transistor being connected to the junction ofsaid diode means and network and also to the base of said thirdtransistor, the emitter of said third transistor being connected to saidoutput junction, a third resistor connected between the emitter of saidfirst transistor and the base thereof, a fourth resistor connectedbetween the base of said first transistor and the collector of saidsecond transistor, whereby a primary current path exists through theemitter-collector of said first transistor, said diode means and saidresistance network when said first transistor is conducting.
 9. Thecombination of claim 8 in which said resistance network includes first,second and third serially connected resistors, a first thermistorconnected in parallel with said first resistor and a second thermistorconnected in parallel with said third resistor of said resistancenetwork.
 10. A current transmitter comprising a pair of currentterminals, a direct current amplifier connected between said terminalsthrough which amplifier a variable portion of the current flowingthrough said terminals passes and is controlled, a constant currentsource connected between said terminals and in parallel with saidamplifier through which source a second portion of the current flows soas to be regulated thereby, a chopper drive and alternating currentcarrier amplifier connected in parallel with each other and in serieswith said current source, a chopper connected between said chopper driveand alternating current amplifier, a condition responsive bridge networkfor controlling said chopper in accordance with the magnitude of avariable condition, said bridge network being in series with saidcurrent source and in series with said alternating current amplifier tocause the current supplied by said current source to recombine with thecurrent through said direct current amplifier, and a demodulator havingiTs input connected to said alternating current carrier amplifier andits output connected to said direct current amplifier for controllingsaid direct current amplifier, whereby the current through saidterminals will be indicative of the amount of variation occurring in thevariable condition.
 11. In a current transmitter, a constant currentsource having input and output junctions, the source comprising first,second and third transistors, each having a base, collector and emitter,diode means, a temperature responsive resistance network, the emitter ofsaid first transistor being connected to said input junction and thecollector thereof being connected to the anode side of said diode means,the cathode side of said diode means being connected to one side of saidresistance network and the other side of said resistance network beingconnected to said output junction, a first resistor connected betweenthe emitter and collector of said first transistor, a second resistorconnected between the collector of said first transistor and the base ofsaid second transistor and also to the collector of said thirdtransistor, the emitter of said second transistor being connected to thejunction of said diode means and network and also to the base of saidthird transistor, the emitter of said third transistor being connectedto said output junction, whereby a primary current path exists throughthe emitter-collector of the first transistor, the diode means and saidresistance network when said first transistor is conducting.
 12. Thecombination of claim 11 in which said resistance network includes first,second and third serially connected resistors, a first thermistorconnected in parallel with said first resistor and a second thermistorconnected in parallel with said third resistor of said resistancenetwork.
 13. The combination set forth in claim 11 including means inseries with said constant current source and energized solely therebyfor providing a control signal in accordance with the magnitude of avariable condition, and a direct current amplifier in parallel with saidconstant current source and said means in series with said constantcurrent source and connected to said constant current source so thatsaid control signal controls said direct current amplifier.